Heat exchanger for fluid media having unequal surface conductances

ABSTRACT

A heat exchanger module comprises thin heat transfer element strips, arranged in a row, parallel to one another, with their surfaces in opposing spaced apart relationship. Opposing spaced apart wall elements bridge adjacent strips, well inwardly of their ends, to cooperate with the strips in defining a row of short, laterally adjacent flow passages. Elongated inlet and outlet manifolds, each extending along the whole row of flow passages, cause internal fluid to flow through all of the flow passages in parallel. Portions of the strips outward of the wall elements comprise fins across which external fluid flows. Both fluids flow parallel to strip surfaces, hence pressure drops are low.

United States Patent Platell HEAT EXCHANGER FOR FLUID MEDIA 10/1968 Richardson l/l79 HAVING UNEQUAL SURFACE 3,735,810 2/197] Ostbo 165/179 CONDUCTANCES Primary ExaminerManuel A. Antonakas Inventor. Ove Bertll Platell, S1gtuna, Sweden Assistant Examiner Theophil w. Streule JR [73] Assignee: Saab-Scania Aktiebolag, Linkoping, Attorney, Agent, or Firmlra Milton Jones Sweden [22] Filed: Sept. 10, 1973 [57] ABSTRACT [21] Appl. No.: 395,923 A heat exchanger module comprises thin heat transfer element strips, arranged in a row, parallel to one another, with their surfaces in opposing spaced apart re- [30] Forelgn Apphcanon Pnomy Data lationship. Opposing spaced apart wall elements Sept. 13, 1972 Sweden 11835/72 bridge adjacent Strips we" inwardly of their ends, to cooperate with the strips in defining a row of short, [52] :LS. Cl. /179, l65/l75 laterally adjacent flow passages Elongated inlet d [51] lit. Cl F28f 1/42 outlet manifoms each extending along h whole row [58] held of Search 165/173 of flow passages, cause internal fluid to flow through 165/166 all of the flow passages in parallel. Portions of the strips outward of the wall elements comprise fins [56] References C'ted across which external fluid flows. Both fluids flow par- UNITED STATES PATENTS allel to strip surfaces, hence pressure drops are low.

1,508,860 9/1924 Stuart 165/179 3,385,356 5/1968 Dalin 165/179 x 5 Clams, 10 Draw'ng figures r t I '1 I 1 I 5 2; ,j 1; L t 1/ u 5 1 e 2 a l7 1 I 1 I F 1 g L) L e was l's as /7 6 a 5 T 2 5 i 2 9 1 51 Sept. 17, 1974 PAIENIEB SEP] 7 I974 aaaagszs SHEEF 1 0? Field.

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HEAT EXCHANGER FOR FLUID MEDIA HAVING UNEQUAL SURFACE CONDUCTANCES This invention relates to heat exchangers by which heat is transferred from one fluid medium to another; and the invention has more particular reference to a heat exchanger of the general type exemplified by automobile radiators and which is particularly intended for effecting heat transfer between a pair of fluids that have substantially differing surface conductances.

Although the present invention is by no means limited in its application to automobile radiators, the automobile radiator affords a familar example of a heat exchanger that involves the problems and objectives with which the invention is concerned, and, for purposes of exemplification, the invention will therefore be explained in its application to such a device.

Ordinarily an automobile radiator comprises metal tubes arranged in laterally spaced parallel relation to one another, in one or more rows. Engine coolant fluid is circulated through the tubes from a header or inlet manifold communicated with one end of each tube to an outlet manifold to which the tubes open at their other ends. Extending transversely to the tubes are metal fins to which the tubes impart heat that they have abstracted from the coolant fluid. The fins, in turn, transfer such heat to air circulated across them. The fins are usually thin, flat strips that are secured to the tubes at spaced intervals along the tubes, with the surfaces of the fins disposed normal to the tube axes. Cooling air is caused to flow across the fins parallel to their surfaces, in a direction transverse to the tube axes and to the lengths of the fins.

It will be apparent that the coolant liquid circulated through automobile radiator tubes constitutes an internal fluid and that the air circulated across the fins constitutes an external fluid. Thus, in general, any heat exchanger can be regarded as a device for effecting indirect transfer of heat between an internal fluid and an external fluid. In any particular heat exchanger one or both fluids may be either a liquid or a gas (or even a liquid-gas mixture such as a steam-water emulsion), and heat transfer may take place in either direction, that is, either from the internal fluid to the external one (as in an automobile radiator) or vice versa (as in a steam boiler).

To return to the automobile radiator example, the need for cooling fins on the coolant-carrying tubes arises from the fact that heat is more readily transferred between liquid engine coolant and metal than between air and metal; hence a large surface area must be presented to the air to transfer to it all of the heat that has been picked up from the liquid in contact with the relatively smaller area of the interior tube surfaces. In more technical terms, the surface conductance or a-value of engine coolant is substantially higher than that of air. Under the conditions that obtain in an automobile radiator, if the engine coolant is water, its a-value is about twenty times that of air; and if the coolant consists of condensing steam its a-value is even greater than that of water.

In most automobile radiators the spacing between adjacent fins in contact with a tube is on the order of a few millimeters. This can be considered a comparatively large spacing, and to afford sufficient fin area so that all of the heat picked up by the tubes will be transferred to the air, the fins are made relatively wide, their width being on the orders of 4 to 5 cm. Hence the cooling air flowing across the fins transverses a relatively long gas path and consequently undergoes a rather high pressure drop. To offset this pressure drop and insure adequate flow of cooling air, a fan is normally provided.

Great care had to be exercised in the design and manufacture of heretofore conventional automobile radiators and similar heat exchangers to insure that the fins had good heat conductive connections to the tubes and that the fins were spaced at accurately uniform intervals along the lengths of the tubes and were accurately parallel to one another. Inasmuch as the fins must be relatively thin, not only for manufacturing economy but also to enable them to offer minimum resistance to air flow, it was not easy to provide them with connections to the tubes that would insure good heat transfer across the joints. If the fins were not uniformly spaced or accurately parallel, the distribution of air flow across the radiator became uneven and the exchanger received its greatest air flow at those points where heat transfer capacity was poorest.

Since the engine compartment of an automobile is usually crowded, and a radiator must be so located therein that air can flow freely through it, compactness is an important requirement for automobile radiators.

With the heretofore conventional fin spacing and fin area used for automobile radiators, a radiator that fitted the space available for it could effect adequate engine cooling in cold and normally warm weather but tended to be inadequate in hot weather.

In order to achieve some improvement in the rate of heat transfer at the fin surfaces, the fins have sometimes been provided with perforated tongues that increased the turbulence of the air flowing across them. However, this expedient for reducing radiator bulk was not entirely satisfactory because the increased turbulence tended to increase the pressure drop in the air flow across the fins and made necessary a more effective fan.

An alternative expedient for increasing the capacity of the heat exchanger to transfer heat from its metal to the air is to reduce the spacing of the fins, thereby increasing the fin area in relation to the interior surface area of the tubes. If the fins are flat, smooth and truly parallel, and are spaced apart by distances of 0.7 mm or less, the air has perfect laminar flow through the slots between the fins, and there will be a relatively low pressure drop in the air stream through the radiator. If the fin spacing is on the order of 0.5 mm, the surface conductance between the heat exchanger and air is of the same order of magnitude as that of the above mentioned type of heat exchanger with fins arranged for turbulent flow, but of course the air pressure drop across the radiator is substantially lower. With such close spacing of fins, the fins need have a width of only about 5 mm in order for the heat exchanger to have the same total fin area as a comparable unit with the more conventional several millimeter fin spacing and 4 to 5 cm fin width as described above, and such narrow fins thus further contribute to low air pressure drop. Moreover, the heat transfer between a tin and air is greatest at the leading edge of the fin so that the average heat transfer value per unit area of fin surface will be greater with decreasing width of the fins.

However, the advantages of narrow, closely spaced fins in a conventional radiator arrangement are in large part offset or lost because from a practical standpoint narrow fins imply small diameter tubes for the coolant liquid. Such tubes present small interior surface areas to the liquid at which it can give up its heat to them, and they bring about a pressure drop that retards flow of the liquid through them. By reason of both of these factors a radiator with closely spaced narrow fins, if arranged in conventional form, would have a substantially lower heat transfer value between the liquid and the tubes than between the fins and the air flowing across them. Obviously a radiator is fully efficient only when it affords an inherent balance or equality between the rate of heat transfer to it and the rate of heat transfer from it, and any substantial departure from such balance entails a waste of the material of which the unit is made and/or a waste of energy required to move one or both of the heat exchanging fluids through it.

One expedient that has been suggested for increasing the heat transfer capacity between the tube walls and the internal fluid, in order to bring it into balance with a high external heat transfer capacity, is the provision of extended surface at the interiors of the liquidcarrying tubes. However, the internal extended surface arrangements heretofore proposed have had the serious disadvantage that they tended to impede the flow of the internal fluid and were thus more or less selfdefeating.

With the foregoing in mind, it is the general object of the present invention to provide a very compact heat exchanger for effecting transfer of heat between an internal fluid that has a relatively high surface conductance and an external fluid that has a relatively low surface conductance, which heat exchanger has high efficiency both because it permits each of the fluids to be circulated through it with a low pressure drop and because it affords good balance of heat transfer values between the respective fluids and the heat exchanger surfaces.

It is also an object of this invention to provide a heat exchanger having unitary heat transfer elements, each having one portion in heat transferring contact with internal fluid and another portion (or other portions) in heat transferring contact with external fluid, so that heat exchange does not take place across a joint that might impair the efficiency of the device by its poor heat conducting ability.

Another and more specific object of this invention is to provide an inexpensive and easily manufactured heat exchanger of the character just mentioned, having heat transfer elements with outer portions that provide closely spaced narrow fins across which the external fluid can flow with a low pressure drop and a high rate of heat transfer and with inner portions that provide heat transfer surfaces of substantial area that are in contact with the internal fluid and extend substantially in its direction of flow, the heat transfer surfaces in contact with the internal fluid thus being so disposed as to allow that fluid to be circulated through the heat exchanger with a low pressure drop while at the same time affording a rate of heat transfer with the internal fluid that is balanced to the rate of heat transfer with the external fluid.

Another specific object of this invention is to provide a heat exchanger that is exceptionally efficient and therefore unusually compact for its capacity, and thus well suited to automotive applications, which heat exchanger owes its efficiency to an arrangement whereby both the internal and the external fluids are in contact with heat conducting surfaces that are very large relative to the volume of heat exchanger bulk, wherein those surfaces extend in the directions of flow of the fluids so that the fluids are subjected to very low flow resistance in the course of passage through the device, and wherein the fluids can be passed in counterflow relationship to one another for maximum heat transfer efficiency.

A further object of this invention, and one that has seldom, if ever, been attained in prior heat exchangers, is the provision of an inexpensive but highly efficient heat exchanger in which portions of the passage defining means for conducting internal fluid through the device, as well as the joints which hold together the components of the heat exchanging surfaces, need not have any primarily heat-absorbing, heat-conducting or heattransmitting function and can therefore be made of an inexpensive material, such as plastic, that can be a poor conductor of heat.

Still another object of this invention is to provide a heat exchanger comprising a few simple elements, readily assembled with one another in a manner that assures accurate spacing between the fins and accurate parallelism of them, and also assures uniform rates of flow of both the internal fluid and the external fluid through all portions of the heat exchanger.

These objects of the invention are achieved by means of a heat exchanger comprising a plurality of thin, striplike heat transfer elements, arranged in a row, parallel to one another, with their surfaces in opposing spaced apart relationship. Each heat transfer element has an inner portion, intermediate its ends, and the inner portions of the several heat transfer elements in the row cooperate with one another and with other wall defining elements to provide a row of short, laterally adjacent flow passages for internal fluid. Longitudinally extending portions of each heat transfer element, at opposite sides of its inner portion, provide fins across which external fluid flows. Other wall means define manifolds that conduct the internal fluid to and from the row of flow passages.

With these observations and objectives in mind, the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings, which exemplify the invention, it being understood that changes may be made in the specific apparatus disclosed herein without departing from the essentials of the invention set forth in the appended claims.

The accompanying drawings illustrate several complete examples of the invention constructed according to the best modes so far devised for the practical application of the principles thereof, and in which:

FIG. 1 is a fragmentary perspective view, with portions cut away, of a heat transfer module of a heat exchanger embodying the principles of the present invention;

FIG. 2 is a view in front elevation of a complete heat exchanger embodying the principles of the present invention;

FIG. 3 is a top view of the heat exchanger shown in FIG. 2;

FIG. 4 is a fragmentary sectional view on a larger scale, taken on the plane of the line 4-4 in FIG. 2;

FIG. 5 is a partially disassembled fragmentary perspective view, generally comparable to FIG. 1 but illustrating a modified embodiment of the invention;

FIG. 6 is an edge-on view of a group of heat transfer elements of another modified form of the heat exchanger of this invention, arranged substantially in assembled relation to one another;

FIG. 7 is a view generally like FIG. 6 but illustrating still another modified embodiment;

FIG. 8 is a front view of a further modified form of heat transfer module of a heat exchanger of this invention;

FIG. 9 is a view in cross section of the heat transfer module shown in FIG. 8; and

FIG. is a side view of the radiator shown in FIGS. 1 and 2.

Referring now to the accompanying drawings, a heat transfer module 5 of a heat exchanger embodying the principles of this invention comprises a plurality of identical heat transfer elements 6, each in the form of a strip of a metal that has good heat conducting properties. As will shortly appear, the heat transfer element strips 6 are presented edge-on to both internal fluid and external fluid, and hence they should be as thin as possible, for minimum resistance to flow of the fluids. The width and length of the strips are not critical and can be established on the basis of criteria discussed hereinafter.

The heat transfer element strips 6 that comprise each heat transfer module 5 are arranged in a row, parallel to one another and with their surfaces facing one another, as may be readily seen from FIG. 1.

An inner portion 7 of each strip, intermediate its ends, is contacted by an internal fluid circulated through the heat exchanger and serves to guide such fluid in its flow and to transfer heat to or from it. The flow of the internal fluid is denoted by shaded arrows. Outer portions of each heat transfer element strip that extend longitudinally to opposite sides of the inner portion 7 comprise fins 8 across which an external fluid flows. The flow of external fluid is denoted by arrows in unshaded outline.

A pair of opposing spaced apart wall elements 9 and 10 extend from one to the other of each pair of adjacent heat transfer elements and have their opposite edges sealed to the opposing surfaces of those heat transfer elements. The several wall elements 9 and 10 cooperate with the inner portions 7 of the several heat transfer element strips to define a row of laterally adjacent, parallel flow passages 11 for the internal fluid. Each flow passage 11 thus has a rectangular cross section, with two opposite sides defined by heat transfer elements and its other pair of opposite sides defined by the wall elements 9 and 10. Note that the wall elements are joined to each heat transfer element along the junctions of its inner portion 7 with its outer fin portions 8.

The internal fluid, as explained hereinafter, is caused to flow in parallel through all of the flow passages 11 in the row thereof. Since the direction of flow of the internal fluid through each of the flow passages 11 is parallel to the surfaces of the heat transfer elements as well as generally parallel to the inner surfaces of the wall elements 9 and 10, those surfaces interpose no substantial resistance to the flow of that fluid. Furthermore, each of the flow passages 11 is axially short (its length being equal to the width of the heat transfer element strip), and in this respect, too, the flow passages present minimal resistance to the flow of the internal fluid. Nevertheless, it will be observed that by reason of the close spacing of the heat transfer elements, and the large number of them in the row, they collectively present to the internal fluid a large area of contact surface at which heat transfer can take place.

Since internal fluid flows in the same direction through all of the flow passages 11 in a row thereof, each flow passage can be considered to have an inlet end (its right-hand end in FIG. I) and an outlet end.

Internal fluid is conducted to the inlet ends of all of the flow passages in a row thereof by means of an elongated inlet manifold 13 defined by a wall member 14 that spans the entire row of flow passages. Having regard to the U-shaped cross-section of the inlet manifold, the tips of the legs of the U are sealingly joined to the respective wall elements 9 and 10, at the inlet ends of the flow passages, and the wall member 14 can thus be said to bridge the wall elements 9 and 110. The interior of the inlet manifold is of course in open communication with the inlet ends of all of the flow passages 11 by reason of the U-shaped cross-section of the wall member 13 that defines it.

Adjacent to one end of the row of flow passages the inlet manifold has in it a fluid inlet 16. At its other end the inlet manifold is closed or dead-ended, as at 17. Preferably the internal cross section of the inlet manifold is tapered or convergent along its length, as best seen in FIG. 4, so as to progressively decrease from the inlet 16 to the closed end. It will be evident that internal fluid entering the inlet 16 in the inlet manifold and tending to flow lengthwise therein is constrained to enter and flow through the low passages, and that the taper of the manifold induces such fluid to flow through all of the flow passages at substantially equal rates.

Fluid issues from the outlet ends of the several flow passages 11 into an elongated outlet manifold 19 that can be identical with the inlet manifold. Thus the outlet manifold can be defined by a substantially U-shaped wall member 20 that is sealed to the wall elements 9 and 10 at the outlet ends of the flow passages, and the length of the outlet manifold is such that it spans the entire row of flow passages.- The outlet manifold has in it a fluid outlet 22 from which internal fluid can be circulated back to the system in which it is used or can be passed to another heat transfer module for further modification of its temperature. Preferably the fluid outlet 22 is located at the end of the outlet manifold that is remote from the fluid inlet 16, while the opposite end of the outlet manifold is closed or dead-ended as at 23. Preferably, too, the interior of the outlet manifold is divergent, increasing uniformly in cross section along its length towards its end having the fluid outlet 22, so as to cooperate with the taper or convergence of the inlet manifold in encouraging internal fluid to flow through all of the flow passages 11 at equal rates.

With the bove described arrangement of the manifolds, the internal fluid makes two right-angle bends in flowing first lengthwise in one direction through the inlet manifold, then substantially at right angles to that direction through the flow passages 11, then again in the first mentioned direction in flowing through the outlet manifold to the outlet 22. But the total cross sectional area of the several flow passages in the row is very large, so that the rate of flow therethrough is comparatively small, and the close spacing of the heat transfer elements tends to maintain laminar flow through the flow passages. Hence, despite the right angle bends in the internal fluid flow path, the internal flow resistance afforded by the heat exchanger of this invention is so low as to be negligible.

Inasmuch as the outer or fin portions 8 of the striplike heat transfer elements have their surfaces parallel to the surfaces of the inner portions 7 that define the flow passages 11, the external fluid flows through the heat exchanger in directions parallel to the direction of flow of the internal fluid through those passages. Consequently the heat exchanger of this arrangement makes possible a desirable counterflow between the internal and external fluids.

It is noteworthy that the direction of flow of the external fluid is such that the manifold defining wall members 14 and 20 can extend any desired distances in the directions edgewise of the strips 6 without affecting the flow of the external fluid. This is to say that the interior volumes of the manifolds can be made as large as desired, to insure slow flow of the internal fluid therethrough and hence low pressure drop, without in anywise compromising flow of the external fluid or the heat exchange efficiency of the module.

It is also noteworthy that the portion 7 of each heat transfer element that is in heat transferring contact with the internal fluid is integral with its fin portions 8 that have heat transferring contact with the external fluid, so that efficiency cannot be compromised by joints across which heat conduction must take place. Furthermore, since only the heat transfer elements 6 are relied upon to effect heat exchange, other portions of the heat transfer module (namely the wall elements 9 and 10 and the manifolds 13 and 19) need not be made of heat conducting material but can instead be made of a relatively inexpensive and easily worked material such as plastic.

In the embodiment of the invention illustrated in FIG. 1, the wall elements 9 and 10 and the manifolds 13 and 19 are depicted as integral portions of a unitary tube 27 of elliptical cross section. The major axis of the ellipse is oriented width-wise of the heat transfer elements and is substantially longer than their widths. Thus the smaller radius portions of the tube provide the wall members 14 and of the inlet and outlet manifolds.

The tube 27 can be of plastic and can have transverse slots in which the flat heat transfer elements 6 are closely received. In assembly of the module, each heat transfer element is slid endwise into the slots intended for it and can be sealed to the tube by means of a suitable cement or the like, deposited all around each slot. With an elliptical tube of uniform cross section along its length, the taper for the inlet and outlet manifolds is provided by arranging the heat transfer elements in such edgewise offset relation to one another that a line connecting their geometrical centers defines a shallow X with the longitudinal centerline of the tube.

In the embodiment of the invention illustrated in FIGS. 8 and 9, the heat transfer elements 6 are again perfectly flat, and the wall elements 9 and 10 and the manifolds l3 and 19 are again defined by portions of a tube having an elliptical cross section; but in this case, instead of the tube being unitary, it is made up of a plurality of axially short elliptical rings 27'. The module comprises alternating rings 27' and heat transfer elements 6, with cement or a similar bonding agent securing and sealing the flat axially opposite surfaces of each ring to the opposing surfaces of its adjacent heat transfer elements as at 29. It would appear that there might be a gap between those portions of adjacent rings that extend beyond the edges of a heat transfer element between them, but in practice the heat transfer elements can be made so thin that reasonably heavy coatings of the bonding material on the rings will seal the rings directly to one another, with the heat transfer elements in effect swallowed up in the joints.

In the embodiment of the invention illustrated in FIG. 5, the wall elements 9 and 10 are integral with the heat transfer element strips 6'. Each such wall element is defined by a pleated portion 9, 10 of the strip that extends generally normal to its inner portion 7 and its outer fin portions 8. By reason of such pleated wall element portions, the inner portion 7 of each strip is in flatwise offset relation to its outer fin portions 8, but the surfaces of those portions are nevertheless parallel to one another. It will be apparent from FIG. 5 that the heat exchanger there illustrated can be assembled by bonding a row of the heat transfer elements 6 directly to one another, as with an epoxy or similar cement. U- shaped wall members 14 and 20, which comprise the manifolds 13 and 19 and which can be made of plastic, can be bonded directly to the inlet and outlet edges of the pleated wall elements in a manner that will be obvious from the figure. Note that the pleat-like wall elements 9' and 10' provide a considerable amount of surface area of heat conductive material that is in contact with both the internal fluid and the external fluid to participate in heat exchange between the fluids.

In the embodiment of the invention illustrated in FIG. 6, which depicts further modified heat transfer elements 6" as seen at their longitudinal edges, the wall elements 9 and 10 are again integral with the heat transfer element strips. In this case the inner portion 7 of each strip is coplanar with its outer fin portions 8, owing to the fact that the bent portions of the strip that define the wall elements are of generally U-shape. However, the legs of the U have outward offsets intermediate their ends, as at 30, so that the U has a narrow portion 31 near its bight and a wider portion 32 outwardly of the offsets 30. For clarity, FIG. 6 exaggerates the amount of offset in the legs of the U. With this in mind, it will be evident from the figure that the narrow portion 31 of each U can be snugly received in the wider portion 32 of the U on an adjacent strip so that the wall element portions of the several heat transfer elements can be readily bonded to one another with a strong connection and a good seal.

FIG. 7 illustrates a further modification that carries forward the principles of the FIG. 6 embodiment. In FIG. 7 the inner portion 7 of each heat transfer strip is in flatwise offset relation to its coplanar outer fin portions 8, owing to a U-shaped bend in the strip. The inner portion 7 comprises the bight of the U, and the legs of the U are formed by the wall element portions 9" and 10" of the strip, each of which has a more or less sinuous pleat that provides an outward offset 30'. It will be apparent from the figure that the U-shaped portions of adjacent heat transfer element strips partially nest in one another to provide for secure connections and good seals.

FIGS. 2, 3 and 10, taken with FIG. 4, illustrate how heat transfer modules 5 like those described above can be combined with one another to comprise a complete heat exchanger suitable for automotive radiator applications. The heat exchanger has a horizontal inlet duct 41 for internal fluid at its upper rear, and has a horizontal outlet duct 42 for such fluid at its lower front. There is an inlet 44 for internal fluid at one end of the inlet duct 41 and an outlet 45 at the end of the outlet duct 42 that is remote from the inlet 44.

At regular intervals rear downcomers 47 extend downwardly from the inlet duct and are of course communicated with its interior so that fluid flowing into the inlet duct is by it distributed to the rear downcomers. Front downcomers 46 similarly extend down to the outlet duct 43.

Each rear downcomer 47 is communicated with one of the front downcomers 46 through a plurality of the heat transfer modules of this invention, said modules extending horizontally between the downcomers they connect and being disposed at regularly spaced intervals along the lengths of the downcomers. As will be evident from FIG. 3, and also from FIG. 4, the front and rear downcomers are arranged in laterally offset, staggered relation to one another so that the heat transfer modules that connect the downcomers are arranged in a zigzag pattern as viewed from the top of the heat exchanger.

Since the surfaces of the fins 8 extend transversely to the longitudinal centerlines of the several modules 5, as best seen in FIG. 4, the external fluid (air, in the case of a radiator) makes two bends as it flows through the heat exchanger, first to change its direction from straight rearward to obliquely rearwardly between the fins, and then to resume its rearward flow. Because of the close spacing of the fins and the short surfaces they present to. the external fluid, as measured along the di rection of its flow, the flow of that fluid is laminar, as explained above, and is therefore attended by a low pressure drop.

It will be apparent that with a heat exchanger arranged as just described, the heat transfer element strips 6 can be made with a length substantially equal to the height of the entire heat exchanger, with each heat transfer element strip extending through all of the vertically superimposed modules 5 that are connected between a front and a rear downcomer, to have several inner portions 7. The distance between modules will then be so chosen as to assure optimum fin area for transfer of heat with the external fluid at the rate desired. The rate of transfer of heat between the internal fluid and the strips can be balanced to the rate of external heat transfer by proper selection of the area of the inner portions 7 of the heat transfer elements. Increase of the area of those portions need not entail increasing the width of the strips beyond that at which laminar flow of both the internal and external fluids is assured, since the area of that inner portion also depends upon the height of the flow passages 11 as measured in the direction lengthwise of the strips.

From the foregoing description taken with the accompanying drawings it will be apparent that this invention provides a heat exchanger which is unusually compact and inexpensive for its capacity and which can have its internal and external rates of heat transfer well balanced in relation to one another to insure optimum heat transfer capacity under a wide range of ambient conditions.

Those skilled in the art will appreciate that the inventioncan be embodied in forms other than as herein disclosed for purposes of illustration.

The invention is defined by the following claims: 1. A heat exchanger by which heat can be transferred between an internal fluid and an external fluid, said heat exchanger comprising: 1

A. a plurality of thin, strip-like heat transfer elements arranged in a row, parallel to one another, with their surfaces in opposing, spaced apart relationship, each of said heat transfer elements having 1. a substantially flat inner portion, to be contacted by internal fluid for heat exchange therewith, and which inner portion cooperates with the inner portions of adjacent heat transfer elements in guiding internal fluid for flow in a direction transverse to the row and to the lengths of the heat transfer elements, and

2. at least one outer portion that extends longitudinally from said inner portion, the surfaces of said outer portion being substantially parallel to those of the inner portion and being adapted to be contacted by external fluid for heat exchange there with, the adjacent outer portions of the heat transfer elements thus being cooperable with one another to guide external fluid for flow in a direction parallel to said first mentioned direction;

B. means defining a pair of opposing, spaced apart wall elements extending between each pair of adjacent heat transfer elements, the several wall elements cooperating with the inner portions of the heat transfer elements to define a row of laterally adjacent flow passages through which internal fluid is constrained to flow in the first mentioned direction, each of said flow passages thus having an inlet end and an outlet end;

c. wall means defining an elongated inlet manifold extending entirely across said row of flow passages, said wall means being sealed to each pair of wall elements at the inlet end of the flow passage defined thereby and having its interior in open communication with the inlet ends of all of said flow passages and with a fluid inlet in the inlet manifold near one end thereof, the inlet manifold having its opposite end closed so that internal fluid entering the fluid inlet and flowing toward said opposite closed end can enter and flow through said flow passages; and

D. wall means defining an elongated outlet manifold extending entirely across said row of flow passages, said wall means being sealed to each pair of wall elements at the outlet end of the flow passage defined thereby, the interior of said outlet manifold being in open communication with the outlet ends of all of said flow passages and with a fluid outlet in the outlet manifold so that the outlet manifold conducts internal fluid flowing through said flow passages to said fluid outlet.

2. The heat exchanger of claim 1, further characterized by:

the interior of the inlet manifold being of decreasing cross section along its length in the direction from the fluid inlet toward the closed opposite end thereof to encourage diversion of fluid therefrom into all of the flow passages.

3. The heat exchanger of claim 1, further characterized by:

said fluid outlet in the outlet manifold being near the end thereof that is remote from the fluid inlet in the inlet manifold, and the opposite end of the outlet manifold being closed, so that internal fluid is constrained to flow lengthwise through the interior of the outlet manifold in a direction opposite to that in which it tends to flow through the inlet manifold.

4. The heat exchanger of claim 3, further characterized by:

each of said manifolds having its interior of decreasing cross sectional area along its length in the direc- 

1. A heat exchanger by which heat can be transferred between an internal fluid and an external fluid, said heat exchanger comprising: A. a plurality of thin, strip-like heat transfer elements arranged in a row, parallel to one another, with their surfaces in opposing, spaced apart relationship, each of said heat transfer elements having
 1. a substantially flat inner portion, to be contacted by internal fluid for heat exchange therewith, and which inner portion cooperates with the inner portions of adjacent heat transfer elements in guiding internal fluid for flow in a direction transverse to the row and to the lengths of the heat transfer elements, and
 2. at least one outer portion that extends longitudinally from said inner portion, the surfaces of said outer portion being substantially parallel to those of the inner portion and being adapted to be contacted by external fluid for heat exchange therewith, the adjacent outer portions of the heat transfer elements thus being cooperable with one another to guide external fluid for flow in a direction parallel to said first mentioned direction; B. means defining a pair of opposing, spaced apart wall elements extending between each pair of adjacent heat transfer elements, the several wall elements cooperating with the inner portions of the heat transfer elements to define a row of laterally adjacent flow passages through which internal fluid is constrained to flow in the first mentioned direction, each of said flow passages thus having an inlet end and an outlet end; c. wall means defining an elongated inlet manifold extending entirely across said row of flow passages, said wall means being sealed to each pair of wall elements at the inlet end of the flow passage defined thereby and having its interior in open communication with the inlet ends of all of said flow passages and with a fluid inlet in the inlet manifold near one end thereof, the inlet manifold having its opposite end closed so that internal fluid entering the fluid inlet and flowing toward said opposite closed end can enter and flow through said flow passages; and D. wall means defining an elongated outlet manifold extending entirely across said row of flow passages, said wall means being sealed to each pair of wall elements at the outlet end of the flow passage defined thereby, the interior of said outlet manifold being in open communication with the outlet ends of all of said flow passages and with a fluid outlet in the outlet manifold so that the outlet manifold conducts internal fluid flowing through said flow passages to said fluid outlet.
 2. The heat exchanger of claim 1, further characterized by: the interior of the inlet manifold being of decreasing cross section along its length in the direction from the fluid inlet toward the closed opposite end thereof to encourage diversion of fluid therefrom into all of the flow passages.
 2. at least one outer portion that extends longitudinally from said inner portion, the surfaces of said outer portion being substantially parallel to those of the inner portion and being adapted to be contacted by external fluid for heat exchange therewith, the adjacent outer portions of the heat transfer elements thus being cooperable with one another to guide external fluid for flow in a direction parallel to said first mentioned direction; B. means defining a pair of opposing, spaced apart wall elements extending between each pair of adjacent heat transfer elements, the several wall elements cooperating with the inner portions of the heat transfer elements to define a row of laterally adjacent flow passages through which internal fluid is constrained to flow in the first mentioned direction, each of said flow passages thus having an inlet end and an outlet end; c. wall means defining an elongated inlet manifold extending entirely across said row of flow passages, said wall means being sealed to each pair of wall elements at the inlet end of the flow passage defined thereby and having its interior in open communication with the inlet ends of all of said flow passages and with a fluid inlet in the inlet manifold near one end thereof, the inlet manifold having its opposite end closed so that internal fluid entering the fluid inlet and flowing toward said opposite closed end can enter and flow through said flow passages; and D. wall means defining an elongated outlet manifold extending entirely across said row of flow passages, said wall means being sealed to each pair of wall elements at the outlet end of the flow passage defined thereby, the interior of said outlet manifold being in open communication with the outlet ends of all of said flow passages and with a fluid outlet in the outlet manifold so that the outlet manifold conducts internal fluid flowing through said flow passages to said fluid outlet.
 3. The heat exchanger of claim 1, further characterized by: said fluid outlet in the outlet manifold being near the end thereof that is remote from the fluid inlet in the inlet manifold, and The opposite end of the outlet manifold being closed, so that internal fluid is constrained to flow lengthwise through the interior of the outlet manifold in a direction opposite to that in which it tends to flow through the inlet manifold.
 4. The heat exchanger of claim 3, further characterized by: each of said manifolds having its interior of decreasing cross sectional area along its length in the direction towards its closed end, to encourage flow of internal fluid at equal rates through all of said flow passages.
 5. The heat exchanger of claim 1, further characterized by: said wall elements being defined by pleat-like bent portions of the heat transfer elements that are intermediate their said inner and outer portions and which extend out of the planes of said inner and outer portions. 